It probably signified #3 of 8 items.Like if you had 8 of 1 tool you could say employee joe smith has 3/8 it is a way of keeping track of tools loaned out to employees.In my experience it is known as a tool tag.
I agree with Steve, it's most likely a tool tag.
I've found a number of them in farm fields over the years... if farmers didn't scratch their initials in the tools, they tagged em!